While the induced reactivation of organophospate-inhibited acetylcholinesterase (AChE) has been intensively studied, hydrolysis of the inactive adduct by water, i.e., spontaneous reactivation, has received relatively little attention. We have found that the conventional preparations of electric eel AChE contain enzyme species that are resistant to aging and reactivate rapidly following exposure to several "irreversible" inhibitors. This proposal outlines a program in which the rapidly reactivating AChE component(s) would be used to study the basis of spontaneous reactivation after treatment with diisopropylphosphorofluoridate and other inhibitors. The reactivation kinetics themselves and their perturbation by various cationic ligands would be examined. Initially, preparations of electric eel and human erythrocyte AChE in which the bulk non-reactivating enzyme had been inhibited would be used. Later, the rapidly reactivating component(s) would be purified from these sources and used in studies that compared the subunit heterogeneity and active site geometry of the reactivating and non-reactivating components. While this study will be concerned mostly with elucidating the mechanism of spontaneous reactivation of AChE, acceleration of the rate of spontaneous reactivation of inhibited AChE could be useful in therapy of organophosphate intoxication.